The present invention relates to electroluminescent devices, and more particularly to electroluminescent devices that utilize a phosphorescent-doped mixed layer architecture for enhanced stability and efficiency.
Organic light emitting devices (OLEDs), which make use of thin film materials that emit light when excited by electric current, are expected to become an increasingly popular form of flat panel display technology. This is because OLEDs have a wide variety of potential applications, including cell phones, personal digital assistants (PDAs), computer displays, informational displays in vehicles, television monitors, as well as light sources. Due to their bright colors, wide viewing angle, compatibility with full motion video, broad temperature ranges, thin and conformable form factor, low power requirements and the potential for low cost manufacturing processes, OLEDs are seen as a future replacement technology for cathode ray tubes (CRTs) and liquid crystal displays (LCDs), which currently dominate the growing $40 billion annual electronic display market. Due to their high luminous efficiencies, electrophosphorescent OLEDs are seen as having the potential to replace incandescent lamps, and perhaps even fluorescent lamps, for certain types of applications.
There are presently three predominant types of OLED construction that are contemplated for these devices: the xe2x80x9cdouble heterostructurexe2x80x9d (DH) OLED, the xe2x80x9csingle heterostructurexe2x80x9d (SH) OLED, and the single layer polymer OLED. In the DH OLED, as shown in FIG. 1A, a substrate 10 is coated by an electrode layer 11, which is typically an anode layer. A thin (typically 100-500 xc3x85) organic hole transporting layer (HTL) 12 is deposited on the anode 11. Deposited on the surface of the HTL 12 is a thin (typically, 50-500 xc3x85) emission layer (EL) 13. The EL 13 provides the recombination site for electrons injected from a thin (typically 100-500 xc3x85) electron transporting layer (ETL) 14 with holes from the HTL 12. Examples of prior art ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870, the disclosure of which is incorporated herein by reference. Such heterostructures may be characterized as having a heterojunction at the HTL/ETL interface.
The device shown in FIG. 1A is completed by the deposition of metal contacts 15, 16 and an electrode layer 17, which is typically a cathode layer. Contacts 15 and 16 are typically fabricated from indium or Ti/Pt/Au. The electrode 17 is often a cathode having a dual layer structure comprising an alloy such as Mg/Ag 17xe2x80x2 directly contacting the organic ETL 14, and an opaque, high work function metal layer 17xe2x80x3 such as gold (Au) or silver (Ag) on the Mg/Ag. A LiF/Al cathode may also be used. When proper bias voltage is applied between the top electrode 17 and the contacts 15 and 16, light emission occurs from the emission layer 13 through, for example, the substrate 10.
The SH OLED, as shown in FIG. 1B, makes use of a multifunctional layer 12 that serves as both EL and HTL or a multifunctional layer 13xe2x80x2 that serves as both EL and ETL. One requirement of the device of FIG. 1B is that the multifunctional layer 12 or 13xe2x80x2 must have a good hole or electron transport capability, respectively. Otherwise, a separate EL layer should be included as shown for the device of FIG. 1A.
Though single and double heterostructures may include one or more polymeric layers, such heterostructures are typically comprised only of thin films of what are commonly referred to as organic small molecule materials. Such organic small molecule materials may be distinguished from polymeric materials simply as being non-polymeric materials.
In contrast to the heterostructures that are typically comprised of two or more thin films only of the small molecule materials, electroluminescent polymer films may typically be incorporated in an OLED having a single organic layer. A single layer polymer OLED is shown in FIG. 1C. As shown, this device includes a substrate 1 coated by an anode layer 3. A thin organic layer 5 of spin-coated polymer, for example, is formed over the anode layer 3, and provides all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer 6 is formed over organic layer 5. The metal is typically Mg or other conventionally-used low work function metals.
Light emission from OLEDs is typically via fluorescence or phosphorescence. As used herein, the term xe2x80x9cphosphorescencexe2x80x9d refers to emission from a triplet excited state of an organic molecule. Successful utilization of phosphorescence holds enormous promise for organic electroluminescent devices. For example, an advantage of phosphorescence is that all excitons (formed by the recombination of holes and electrons in an EL), which are formed either as a singlet or triplet excited states, may participate in luminescence. This is because the lowest singlet excited state of an organic molecule is typically at a slightly higher energy than the lowest triplet excited state. This means that, for typical phosphorescent organometallic compounds, the lowest singlet excited state may rapidly decay to the lowest triplet excited state from which the phosphorescence is produced. In contrast, only a small percentage (about 25%) of excitons in fluorescent devices are capable of producing the fluorescent luminescence that is obtained from a singlet excited state. The remaining excitons in a fluorescent device, which are produced in the lowest triplet excited state of an organic molecule, are typically not capable of being converted into the energetically unfavorable higher singlet excited states from which the fluorescence is produced. This energy, thus, becomes lost to radiationless decay processes that only tend to heat-up the device.
As a consequence, since the discovery that phosphorescent materials can be used as the emissive material in highly efficient OLEDs, there is now much interest in finding still more efficient electrophosphorescent materials and OLED structures containing such materials.
The present invention includes light emitting devices with a phosphorescent-doped mixed layer architecture. These light emitting devices comprise a substrate, an anode layer; a hole injecting layer over the anode layer; a mixed layer over the hole injecting layer, the mixed layer comprising a hole transporting material and an electron transporting material and being doped with a phosphorescent material; and a cathode layer over the phosphorescent-doped mixed layer.
The mixed layer utilized in the devices of the present invention serves as the emission layer, wherein the hole transporting material and the electron transporting material in the mixed layer act as the host material for the phosphorescent dopant. This mixed layer serves to substantially reduce the accumulation of charge that is normally present at the heterojunction interface of heterostructure devices, thereby reducing organic material decomposition and enhancing device stability and efficiency. Such OLEDs having the mixed layer of the present invention may to a certain extent be thought of as xe2x80x9csingle-layerxe2x80x9d devices, that is, to the extent that they may not have the more strongly charged heterojunction interface that is typical of a heterostructure device.
Although most holes recombine with electrons within the emissive layer, there is a possibility that excess holes may migrate to the cathode and become neutralized. This may sometimes be referred to as hole quenching. So as to prevent or reduce this neutralization or quenching of holes, an additional electron transporting layer may be provided between the mixed layer and the cathode. The electron transporting layer serves to block migration of excess holes to the cathode, thereby keeping the holes within the mixed layer to enhance device stability and efficiency. It also serves as an exciton blocking layer to confine the excitons in the emissive layer.
So as to reduce loss of excitons in the opposite direction, that is, into the hole injecting layer, a hole transporting layer may be present between the mixed layer and the hole injecting layer.
An advantage of the present invention is that it provides exceptionally stable organic light emitting devices with high device efficiency.
As used herein, the following phrases and terms have the following meanings:
xe2x80x9cAlq3xe2x80x9d refers to tris-(8-hydroxyquinoline) aluminum.
xe2x80x9cAnodexe2x80x9d refers to a positively charged electrode for injecting holes into an OLED.
xe2x80x9cBTPIrxe2x80x9d refers to bis(2-(2xe2x80x2-benzo[4,5-a]thienyl)pyridinato-N,C3xe2x80x2)iridium(III)acetylacetonate.
xe2x80x9cCathodexe2x80x9d refers to a negatively charged electrode that injects electrons into an OLED.
xe2x80x9cCBPxe2x80x9d refers to 4,4xe2x80x2-N,Nxe2x80x2-carbazole biphenyl.
xe2x80x9cCuPcxe2x80x9d refers to copper phthalocyanine.
xe2x80x9cELxe2x80x9d refers to, xe2x80x9cemission layer,xe2x80x9d which is a layer in which holes and electrons recombine to produce excitons, which may emit light, or which may transfer their energy to a dopant molecule which emits light, when a proper bias voltage is applied across the device.
xe2x80x9celectron transporting materialxe2x80x9d refers to a charge carrying material for which charge transport is predominantly electron transport.
xe2x80x9cETLxe2x80x9d refers to an xe2x80x9celectron transporting layer,xe2x80x9d which is a layer comprising an electron transporting material as the only type of charge carrying host material.
xe2x80x9cheterojunction devicesxe2x80x9d refer to typical single heterostructure or double heterostructure OLEDs in which an HTL/ETL interface is present.
xe2x80x9cHILxe2x80x9d refers to xe2x80x9chole injecting layer,xe2x80x9d which comprises a hole injecting material to enhance the hole injection efficiency of the anode.
xe2x80x9chole transporting materialxe2x80x9d refers to a charge carrying material for which charge transport is predominantly hole transport.
xe2x80x9cHTLxe2x80x9d refers to a xe2x80x9chole transporting layer,xe2x80x9d which is a layer comprising a hole transporting material as the only type of charge carrying host material.
xe2x80x9cmixed layerxe2x80x9d refers to a layer that comprises a hole transporting material and an electron transporting material.
xe2x80x9cNPDxe2x80x9d refers to N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis-alpha-napthylbenzidine.
xe2x80x9cPtOEPxe2x80x9d refers to 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphine platinum II.